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Do YOU get cramp? This is what causes the painful condition - and how to get rid of it

When the modified bacteria interacts with tetrathionate in the intestines, it switches on an enzyme.

That enzyme is then passed in stools, along with the bacteria.

The researchers were then able to identify the presence of the enzyme, because it changes colour from white to blue.

While the current tests only allow the colour change to be seen under a microscope, the scientists hope that one day fluorescent genes could be used to allow people to spot changes in their own faeces.